Methanol is an excellent hydrogen and energy source for fuel cells. Methanol's utility derives from its relative ease of storage and transportation as well as the relative ease with which it can be converted to H2 using a reformation reactor. In the reformation reactor, hydrogen is produced from methanol using a metal catalyzed steam reformation process. According to the chemistry of the reaction, and under appropriate conditions, methanol and water are reacted to form hydrogen and carbon dioxide.CH3OH+H2O→CO2+3H2  (1)
One drawback of the reformation process is the formation of by-product CO through a pathway distinct from hydrogen production. This by-product of the reformation process must be removed or “scrubbed” from the product H2 prior to introduction into a given fuel cell. Typically, this scrubbing process is achieved through water gas shift reaction. Alternatively, CO may be separated using a Pd or Pd alloy membrane. Pd and Pd alloy membranes, though, require high operating temperatures to be effective; thus, the reformer must be operated at a higher temperature, typically at about 400° C. Such high temperatures, however, require catalysts having excellent durability given that high temperature frequently adversely affects catalyst life.
The utilities of currently available catalyst are affected by these limitations. For example, commercially available copper-zinc catalysts (CuO—ZnO/Al2O3) cannot be used at temperatures above 250° C. due to sintering and deactivation. What is more, these catalysts need to be reduced prior to use in a reformer. A further limitation of copper-zinc catalysts is that they are pyrophoric, creating handling and storage issues.
Various attempts to improve CuO—ZnO catalyst formulations have been described in for example U.S. Pat. No. 6,576,217, as well as U.S. Patent Publication Nos. 2002/0051747, 2002/0169075, and 2004/0006915. The various improvements proposed in the noted documents include introduction of Zr, Al, Ce, or rare earth elements to the copper-zinc formulation.
Substitution of different elements for either Zn or Cu have also been proposed. For example, in U.S. Patent Publication No. 2004/0006915, a catalyst containing ZnO and chromium oxide but not copper oxide was disclosed for use for methanol steam reforming. Other modifications to copper-zinc catalysts include the substitution of Pd for Zn, as discussed in JP Application No. 2002263499; the substitution of Ag for Cu as discussed in JP Application No. 2003154270; the substitution of Cr for Cu as discussed in JP Application No. 2003160304; and the substitution of SiO2 for Zn as discussed in JP Application No. 2008043884. These formulations, however, have not resolved the inherent limitations present in Zn, Cu, or CuZn derived catalysts.
Recently, a non-CuZn base-metal catalyst, alumina supported FeCoNi, was described in U.S. Patent Publication No. 2007/0294942. This catalyst, however, requires expensive organometallic complexes and template molecules to achieve the desired crystal size, thus limiting its utility. Moreover, like copper-zinc catalysts, this base-metal based catalyst is prone to deactivation at high temperatures.
Various other Zn based methanol reformation catalysts are known. For example, U.S. Pat. No. 4,613,584 describes the utility and the formation of PdZn and PtZn alloys for methanol steam reforming. See also Iwasa et al., Applied Catalysis A: General, 1995, 125(1): 145-157 as well as U.S. Pat. No. 6,413,449.
PdZnZr based catalysts are described in U.S. Patent Publication No. 2001/0021469, while Pd/Pt—CuZn catalysts are described in U.S. Patent Publication No. 2002/0039965. These catalyst, though, also suffer from deactivation. For example, deactivations of alumina supported PdZn catalysts have been reported at 285° C. (see Pfeifer et al, Applied Catalysis A: General, 270 (1-2), 165-175, 2004) and even at 250° C. (see Kim, T., et al, Journal of Power Sources, 2006, 155, (2), 231-238). PdZnZr catalyst is further plagued by the potential leaching of Zn during the reforming process. The leached Zn may damage any separation membrane present, as well as the fuel cell itself.
Other known alloys suitable for methanol steam reformation include ZrCu, ZrAu, HfCu, ZrCo, and YNi. Despite their utility, these catalysts are difficult to prepare and require melting metal salt precursors. See e.g., U.S. Pat. No. 5,635,439.
Pd—Ga, Pd—In, Pt—Ga and Pt—In catalysts (Iwasa, Catalyst Letter 54, 1998, 119-123) as well as Pt—Ce/Fe/La— supported on alumina (U.S. Patent Publication No. 2007/0183968) have likewise been shown to be active for methanol steam reforming. These catalysts, too, require melting metal precursors or multi-step processes involving pre-reduction of Pt followed by sequential loading of other metals.
Thus, although the art describes many catalysts useful for methanol steam reformation, each of the variously known catalysts has at least one characteristic that renders the catalyst less than suitable for large-scale commercial use.
In view of the forgoing, it is an objective of the present disclosure to provide new catalyst formulations and processes for making these catalysts, that alleviate many or all of the failures of currently available methanol steam reforming catalysts. In particular, it is an objective of the present disclosure to provide air-stable catalysts for the reformation of methanol that operate continuously at at least about 350° C. and that are easily prepared without the need for melting or pre-reduction.